Pythagoras enjoined abstinence from the flesh of animals because this was conducive to Peace. Those who are accustomed to abominate the slaughter of other animals, as iniquitous and unnatural, will think it still more unlawful and unjust to kill a man or to engage in war. Especially he exhorted the politicians and legislators to abstain, for if they were willing act justly, in the highest degree, it was indubitably incumbent upon them to not injure any of the lower animals, since how could they persuade others to act justly, if they themselves proved to be indulging an insatiable avidity by devouring those animals allied to us, since through the communion of life and the same elements, and the sympathy existing, they are as it were conjoined to us by a fraternal alliance.

Extract from a review of Animal Minds and Human Morals - The Origins of the Western Debate by Richard Sorabji. Review by Stephen Salkever:

For Sorabji, the pro-animal side of the ancient debate, the side arguing that the gap between human and animal psychology is not so large, is best represented by various Aristotelians (especially Theophrastus, Aristotle's successor as leader of the Peripatos) and Platonists (especially Plutarch and Porphyry). A key figure in Sorabji's history of the fading away of this alternative is Iamblichus, who turned Neoplatonism away from its earlier assertions of a significant kinship between humans and other animals, and so sets the stage for the nearly complete triumph of the anti-animal view.